An accelerometer is one of the primary sensors used in on-board automotive safety control systems and navigational systems, particularly inertial navigational systems. Examples of such automotive applications include anti-lock braking systems, active suspension systems, and seat belt lock-up systems.
More specifically, an accelerometer is a device which measures acceleration, or more accurately, accelerometers measure the force that is exerted by a body as the result of a change in the velocity of the body. A moving body possesses inertia which tends to resist the change in velocity. It is this resistance to any change in velocity that is the source of the force which is exerted by the moving body. This force is directly proportional to the acceleration component in the direction of movement when the moving body is accelerated.
In a conventional accelerometer, a mass is suspended between two spring members which are coaxially attached on opposite sides of the mass The mass is maintained in a neutral position so long as the system is at rest or is in motion at a constant velocity. When the mass-spring system undergoes a change in velocity in the direction of the springs' axis, such as an acceleration or deceleration in that direction, the spring mounted mass will resist the movement because of its inertia. This resistance to the change in velocity will force one of the springs to be in tension while the other spring is compressed. Accordingly, the force acting on each spring is equal but opposite in magnitude. The well-known mathematical interrelationship of the three variables--force, weight and acceleration--provides that the force generated is equal to the product of the weight of the mass and the acceleration of the mass, divided by the gravitational constant.
Silicon-based microaccelerometers having resonant-type microbridges are also known. An example of this type of accelerometer is disclosed in U.S. Pat. No. 4,901,570 to Chang et al, assigned to the assignee of the present invention. Chang et al disclose a microaccelerometer having a central square-shaped proof mass which is suspended by at least one pair of resonant microbridges. The resonant bridges are attached to a supporting substrate which circumscribes the proof mass with a gap provided therebetween. As such, the central proof mass is supported within and has free movement relative to the supporting substrate. The individual microbridges within each pair of microbridges are positioned at opposing edges of the proof mass such that the pair's longitudinal axes constitute a common axis across the surface of the proof mass.
In a microaccelerometer employing resonant microbridges, acceleration in the plane of the substrate causes differential axial loads on oppositely disposed resonant microbridges, i.e., causes one supporting resonant bridge to be in compression and the other in tension. It is the inertial force of the proof mass which generates the axial load on the resonant microbridges. In turn, it is the compressive and tensile loads which produce a shift in the resonant frequencies associated with each resonant microbridge. The resulting difference between the resonant frequencies of the compressive and tensile members can then be measured and used to determine the magnitude of the acceleration component in the direction of the common axis shared by the pair of resonant microbridges.
The microbridges taught by Chang et al are driven electrostatically at their respective resonant frequencies by a separate drive electrode. The maximum amplitude of the vibration of each microbridge occurs when the microbridge is at resonance, whereby the frequency of the drive voltage which is supplied to the drive electrode coincides with the natural frequency of the microbridge. To sustain the microbridge in resonance, any shift in its resonant frequency due to the externally imposed stress of the proof mass must be compensated for by a corresponding change in the frequency of the drive electrode's drive voltage.
The frequency of vibration of each microbridge is detected by monitoring the change in a voltage-induced capacitance between the microbridge and a sensing electrode. The capacitance varies with time according to the frequency of vibration of the microbridge. By placing the sensing electrode in close proximity to the microbridge, the shift in the microbridge's frequency of vibration can be detected.
Since the resulting capacitance is small and stray capacitances are usually much larger than the sensed capacitance, the signal derived from the time-varying capacitance change must be amplified and buffered by an on-chip circuitry, such as a clamping diode in conjunction with a depletion mode n-channel metal-oxide-semiconductor field-effect transistor, or MOSFET. To sustain the microbridge in resonance, this enhanced signal is provided through feedback circuitry to the drive electrode, causing the frequency of the drive voltage to change so that it again coincides with the shifted resonant frequency of the microbridge.
This type of resonant microaccelerometer is attractive for precision measurements because the frequency of a micromechanical resonant structure exhibits good linearity with high sensitivity, resolution, and bandwidth. However, a shortcoming of such structures as that taught by Chang et al is that the gap between the sensing electrode and the corresponding microbridge must be sufficiently small so as to maximize the capacitance being detected. This requirement necessitates the evacuation of the microaccelerometer package so as to reduce the damping effects of the air squeezed between the components, typically referred to as a squeeze film effect.
For purposes of assessing the quality of a vibrating structure as a harmonic oscillator, the art has derived a dimensionless number which is referred to as the quality factor (Q). The quality factor of a given structure is inversely related to the damping factor associated with the structure and generally relates to the sharpness or width of the response curve in the vicinity of the resonant frequency of the vibrating structure. The concepts of quality factor, damping, and resonant frequencies are primary factors when considering the vibrational characteristics of a vibrating structure, and will therefore be referred to and further discussed in relation to the present invention.
With regards to the microaccelerometer taught by Chang et al, the evacuation of the microaccelerometer package is necessary to reduce the damping effects of the air in order to achieve a high quality factor. As an example, testing has indicated that vacuum packaging of approximately 100 mTorr is necessary to achieve a quality factor of 600 with the structure taught by Chang et al. Microbridge resonance cannot be initiated in the structure taught by Chang et al when operated at one atmosphere as a consequence of the high damping effect of the air, and hence a low quality factor.
Another significant shortcoming of the teachings of Chang et al is that the microaccelerometer structure exhibits a relatively high off-axis (which is the axis orthogonal to the plane in which acceleration is being detected) response on the order of 10% as compared to the on-axis response. The off-axis response is attributed to a geometrical mismatch between the paired microbridges. The mismatch itself is created in part by non-ideal microfabrication techniques which produce less than ideal symmetry of the proof mass and bridge dimensions. As a result, the bridges are asymmetrically stressed and therefore have different resonant frequencies. Another factor is the package-induced stresses within the bridges which result from such conditions as less than perfect positioning of the proof mass relative to the supporting substrate and inherent stresses associated with the bonding and packaging techniques employed. As a result, the bridges are further asymmetrically stressed.
The detrimental effects of this geometric mismatch are exacerbated by the resonant frequency measuring technique adopted by Chang et al. For an electrostatic drive as taught by Chang et al, the electrically measured resonant frequency is a strong function of the gap obtained between the bridge and drive electrode, and thus, this effect on the measured resonant frequency is amplified. This sensitivity produces an erroneous shift in the measured resonant frequency of each microbridge as detected by the sensing electrode. As a result, the unequal built-in stresses associated with a pair of microbridges directly contribute to a resonant frequency error which does not cancel out during computation. The result is an erroneous acceleration amplitude reading.
Therefore, it would be advantageous to provide a microaccelerometer which employs silicon integrated circuit technology while minimizing the effects of any geometrical mismatch of the microaccelerometer's construction. In addition, it would be advantageous to provide a microaccelerometer which does not employ an electrostatic driver for driving the microbridges at their resonant frequencies, so as to alleviate the shortcomings associated with the prior art. Finally, it would be desireable to reduce the microaccelerometer package's vacuum requirements while maintaining or improving the microaccelerometer's quality factor.